With limited info at hand, you can try the following after following Fredy's advise, if car is in good condition, then you need to work on your skills.
Follow correct lines on dunes if you are following more powerful off-road toys.
Read the sand and angle of attack to be on the hard side of the dunes and not through slip face.
Hard side has wind signature lines vs slip face is plain side with no lines.
Deflate to 10 PSI, if you are with AT tires. 12 PSI is fine with HT tires.
Restrict the car on 1st gear on steep climbs till 6k rpm redline, in case you are upshifting to 2nd at 4k rpm.
Hope you aren't running oversized wheels and extra heavy tires, which severely restrict climbing capabilities.